This invention relates to a new process for the preparation of 2,2,6,6-tetramethyl-4-oxopiperidine, a known compound, sometimes referred to by the name triacetonamine. Triacetonamine has been recognized as a key intermediate in the preparation of certain 4-substituted 2,2,6,6-tetramethylpiperidine derivatives which are excellent in protecting synthetic resin compositions against the deleterious influences of heat and light. For an outline of how triacetonamine is used as intermediate in the preparation of effective stabilizers the disclosure of M. Minagawa et al in U.S. Pat. No. 4,124,564 of Nov. 7, 1978 can be consulted, particularly Col. 7 line 14 to Col. 9 line 30.
Triacetonamine has been known at least since the work of W. Heinz published in Annalen der Chemie, 1880, vol. 203, page 336. Heinz converted acetone to phorone (2,6-dimethylhepta-2,5-dien-4-one) in about 30% yield and this with ammonia to triacetonamine in 70% yield. H. K. Hall in Journal of the American Chemical Society, 1957, vol. 79, page 5447, described a reaction of acetone with ammonia in the presence of calcium chloride for 7 days that gave about 20% yield of triacetonamine after careful fractional distillation to separate the desired product from a different basic nitrogen compound having a nearby boiling point. R. B. Bradbury et al in Journal of the Chemical Society 1947, pages 1394-99, described reactions of acetone and ammonia, alone and with a number of different catalysts, that did not give any triacetonamine. Bradbury's product, obtained in 17% yield without catalyst and in 35% to 90% yield depending on catalyst choice was 2,2,4,4,6-pentamethyl-2,3,4,5-tetrahydropyrimidine hydrate, split to diacetonamine ##STR1## oxalate by the action of alcoholic oxalic acid.
K. Murayama in U.S. Pat. No. 3,513,170 of May 19, 1970 disclosed the conversion of Bradbury's pyrimidine (acetonine) to triacetonamine by the action of a Lewis acid in presence of water. Murayama's Lewis acids include zinc chloride, calcium chloride, and picric acid. In the same patent, Murayama also disclosed the use of diacetone alcohol or acetone with acetonine and/or ammonia, to prepare triacetonamine, as well as the reaction of triacetonamine with diacetone alcohol and calcium chloride under similar conditions to give a higher condensation product 1,9-diaza-2,2,8,8,10,10-hexamethyl-4-oxospiro(5,5) undecane having an empirical formula C.sub.15 H.sub.28 N.sub.2 O.
I. Orban in U.S. Pat. No. 3,943,139 of Mar. 9, 1976 disclosed preparation of triacetonamine by heating phorone with aqueous ammonia and basic catalysts, such as lithium, sodium, calcium, or barium hydroxide, in an autoclave under pressure.
I. Orban in U.S. Pat. No. 3,953,459 of Apr. 27, 1976 disclosed preparation of triacetonamine from acetonine with acetone or diacetone alcohol either in the presence or in the absence of water and an acidic catalyst such as boron trifluoride in a controlled amount of 0.2 to 12 mole % relative to the acetonin.
I. Orban in U.S. Pat. No. 3,959,295 of May 25, 1978 disclosed preparation of triacetonamine from ammonia and acetone or an acidic self-condensation product of acetone in the presence of acidic catalysts in two stages carried out at two different temperatures, such as about 15.degree. C. in the first stage and 50.degree.-55.degree. C. in the second stage, with the amount of acetone being at least 1.6 moles per mole of ammonia.
K. Murayama in U.S. Pat. No. 3,959,298 of May 25, 1976 disclosed preparation of triacetonamine from acetonine and water in the presence of at least 0.125 mole acid catalyst per mole of acetonin. Catalysts included an acetonin salt, an ammonium salt, an amine salt, a mineral acid, or an organic acid.
I. Orban in U.S. Pat. No. 3,960,875 of June 1, 1976 disclosed the preparation of triacetonamine by heating acetonine with acetone or diacetone alcohol in an alcohol solvent without catalyst.
K. Murayama in U.S. Pat. No. 3,963,730 of June 15, 1976 disclosed preparation of triacetonamine by heating acetonine with acetone under anhydrous conditions using an ammonium or amine salt catalyst such as ammonium chloride, ammonium formate, acetonine hydrochloride, pyridine hydrochloride, a hydrochloric acid treated carboxylic acid type ion exchange resin, acetonineacetate, and urea nitrate.
Consideration of the above art as a whole leaves a confused and contradictory impression, with no clear indication of the nature of the key process variables or directions toward a practically workable process.